I'm not sure where you heard that, or what sort of application they were talking about.

If you want a filehandle watcher in Gtk2, you can do that, Gtk2 is pretty good in my book, although I never had trouble with Tk's speed, unless the number of widgets were very large.

You can run mixed event loops by making one loop a master, and the other a slave. The slave loop gets pumped every so many milliseconds by a timer in the master loop. See Wx with Tk and here below is a Gtk2-Tk hybrid script. It may be useful.

#!/usr/bin/perl -w
use strict;
use Gtk2;
use Tk;

#setup Tk loop
my $mw = MainWindow->new(-title=>'Tk Window');
my $count_tk = 0;
my $labtk = $mw->Label(-textvariable =>\$count_tk)->pack; 

#setup Gtk2 loop
Gtk2->init;
my $count_gtk = 0;
my $window = Gtk2::Window->new('toplevel');
$window->set_title('Gtk2 Window');
my $glabel = Gtk2::Label->new("This is a  Gtk2 Label $count_gtk");
$window->add($glabel);
$window->show_all;

# make Tk loop the master, but you could make Gtk2 master if desired
# the lower the repeat rate, i.e. 1 ms, 
# will give more cpu time to the gtk2 loop
# this is sometimes called manually pumping the event loop
my $tktimer = $mw->repeat(10, sub{      
   $count_gtk++;
   $glabel->set_text("This is a  Gtk2 Label $count_gtk");
   Gtk2->main_iteration while Gtk2->events_pending;
   
   $count_tk++;
  });


$mw->Button(-text=>' Tk control Quit  ',
       -command => sub{exit}
        )->pack();

MainLoop;

########################################

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